Intent to prototype: @starting-style rule in CSS Transitions Level 2
Boris Chiou
Summary: The @starting-style rule defines a grouping rule, and the style rules inside it are used to establish styles to transition from, if the previous style change event did not establish a before-change style for the element whose styles are being computed. We also use the starting style to create the transitions when changing the display property from none. This means it provides an additional way to create CSS transitions even if there is no before-change style (or the before-change style is the same as after-change style when changing display property).
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1834876
Specification: https://drafts.csswg.org/css-transitions-2/#at-ruledef-starting-style
Standards Body: CSSWG
Platform coverage: all
Preference: "layout.css.starting-style-at-rules.enabled"
DevTools bug: No needed for now (or we could file a new bug for it if we would like to show more specific info for starting style),
Link to standards-positions discussion: https://github.com/mozilla/standards-positions/issues/833
Other browsers:
- Blink: Implemented this already. Shipped. https://groups.google.com/a/chromium.org/g/blink-dev/c/nHEvqrT5wvM/m/fCp-Kid8AgAJ
- WebKit: implemented and ready to enable by default: https://bugs.webkit.org/show_bug.cgi?id=268369 (Note: wpt.fyi still shows tests failed on stable but the webkit bug has been resolved.)
web-platform-tests: All tests in "css/css-transitions/starting-style-*.html", and the parsing test is "css/css-transitions/parsing/starting-style-parsing.html".
Thanks.
Boris